Wireless communication systems are widely known in which base stations (BSs) communicate with user equipments (UEs) (also called subscriber or mobile stations) within range of the BSs.
The geographical area covered by a base station is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to form a network covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously). In more advanced systems, the concept of a cell can also be used in a different way: for example to define a set of radio resources (such as a given bandwidth around a carrier centre frequency), with an associated identity which may be used to distinguish one cell from another. The cell identity can be used for example in determining some of the transmission properties of communication channels associated with the cell, such as using scrambling codes, spreading codes and hopping sequences. A cell may also be associated with one or more reference signals (see below), which are intended to provide amplitude and/or phase reference(s) for receiving one or more communication channels associated with the cell. Therefore, it is possible to refer to communication channels associated with a cell being transmitted from or by the cell (in the downlink), or transmitted to a cell (in the uplink), even if the transmission or reception is actually carried out by a base station. Typically, in an FDD system, a downlink cell is linked or associated with a corresponding uplink cell. However, it should be noted that it would in principle be possible to organise a communication system which has cell-like features without explicit cells being defined. For example, an explicit cell identity may not be needed in all cases.
Each BS divides its available bandwidth, i.e. frequency and time resources in a given cell, into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one “serving” cell. For some purposes a BS may also be described as an “access point” or a “transmission point”.
Modern wireless communication systems such as LTE and LTE-A are hugely complex and a full description of their operation is beyond the scope of this specification. However, for assisting understanding of the inventive concepts to be described later, some outline will be given of some of the features of LTE which are of particular relevance in the present invention.
OFDM and OFDMA
OFDM (Orthogonal Frequency Division Multiplexing) is one known technique for transmitting data in a wireless communication system. An OFDM-based communications scheme divides data symbols to be transmitted among a large number of subcarriers, hence the term frequency division multiplexing. Data is modulated onto a subcarrier by adjusting its phase, amplitude, or both phase and amplitude. The “orthogonal” part of the name OFDM refers to the fact that the spacings of the subcarriers in the frequency domain are specially chosen so as to be orthogonal, in a mathematical sense, to the other subcarriers. In other words, they are arranged along the frequency axis such that the sidebands of adjacent subcarriers are allowed to overlap but can still be received without inter-subcarrier interference. As the user equipments will receive the same signals at slightly different timings, in other words with a certain delay spread, each OFDM symbol is preceded by a cyclic prefix (CP), which is used to effectively eliminate inter-symbol interference. Further, OFDM enables broadcast services on a synchronized single frequency network with appropriate cyclic prefix design (see below). This allows broadcast signals from different cells to combine over the air, thus significantly increasing the received signal power and supportable data rates for broadcast services.
When individual subcarriers or sets of subcarriers are assigned to different user equipments, the result is a multi-access system referred to as OFDMA (Orthogonal Frequency Division Multiple Access), as used in LTE and LTE-A for the downlink—in other words for communication from base stations to user equipments. By assigning distinct frequency/time resources to each user equipment in a cell, OFDMA can substantially avoid interference among the users served within a given cell.
In OFDMA, users (called UEs in LTE) are allocated a specific number of subcarriers for a predetermined amount of time. An amount of resource consisting of a set number of subcarriers and OFDM symbols is referred to as a resource block (RB) in LTE. RBs thus have both a time and frequency dimension. Allocation of RBs is handled by a scheduling function at the base station (an eNodeB in an LTE-based system).
In LTE, each eNodeB may have a plurality of antennas, and may serve multiple cells at the same frequency simultaneously. One eNodeB may be considered to comprise one or more BSs. Moreover, there may be distinct uplink and downlink cells (in the remainder of this specification, the term “cell” can be assumed to mean at least a downlink cell). Incidentally, the wireless network is referred to as the “E-UTRAN” (Evolved UMTS Terrestrial Radio Access Network) in LTE. The eNodeBs are connected to each other, and to higher-level nodes, by a backhaul network, e.g. the core network or Evolved Packet Core (EPC).
Frame Structure and Resource Blocks
In a wireless communication system such as LTE, data for transmission on the downlink is organised in OFDMA frames each divided into a number of sub-frames. Various frame types are possible and differ between FDD and TDD for example. Frames follow successively one immediately after the other, and each is given a system frame number (SFN).
FIG. 1 shows a generic frame structure for LTE, applicable to the downlink, in which the 10 ms frame is divided into 20 equally sized slots of 0.5 ms. A sub-frame SF consists of two consecutive slots, so one radio frame contains 10 sub-frames.
The available downlink bandwidth consists of NBW sub-carriers with a spacing of f=15 kHz. In case of multi cell MBMS transmission (see later), a sub-carrier spacing of f=7.5 kHz is also possible. NBW can vary in order to allow for scalable system bandwidth operation up to 20 MHz.
FIG. 2 shows a so-called downlink resource grid for the duration of one downlink slot. One downlink slot consists of Nsymb OFDM symbols in general. To each symbol, the above-mentioned cyclic prefix (CP) is appended as a guard time, as shown in FIG. 1.
Nsymb depends on the cyclic prefix length. The generic frame structure with normal cyclic prefix length contains Nsymb=7 symbols as illustrated in FIG. 2. Additionally, an extended CP is defined in order to cover large cell scenarios with higher delay spread, and for MBMS transmission (see below).
The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM symbols, as shown in FIG. 2. Each element in the resource grid is called a resource element (RE) and each resource element corresponds to one symbol.
OFDMA allows the access by multiple UEs to the available bandwidth as already mentioned. Each UE is assigned a specific time-frequency resource. The data channels are shared channels, i.e. for each transmission time interval of 1 ms, a new scheduling decision is taken regarding which UEs are assigned to which time/frequency resources during this transmission time interval. The basic scheduling unit for allocation of resources to the UEs is called a resource block (RB). As shown in FIG. 2, one resource block is defined as 7 consecutive OFDM symbols in the time domain (or 6 with extended CP) and 12 consecutive sub-carriers in the frequency domain. In one OFDM symbol, the number of subcarriers can be one of 128, 256, 512, 1024, 1536, and 2048. The resource block size is the same for all system bandwidths, therefore the number of available physical resource blocks depends on the bandwidth.
Several resource blocks may be allocated to the same UE, and these resource blocks do not have to be adjacent to each other. Scheduling decisions are taken at the base station (eNodeB). The scheduling algorithm has to take into account the radio link quality situation of different UEs, the overall interference situation, Quality of Service requirements, service priorities, etc.
Reference Signals
To facilitate measurements of the radio link properties by UEs, and reception of some transmission channels, reference signals are embedded in the downlink sub-frame as transmitted from each antenna of an eNodeB or more correctly, “antenna port”. The term “antenna port” is preferred when referring to transmissions from multiple antennas, since it is possible for multiple physical antennas to transmit copies of the same signal and thus act as a single antenna port.
In case of two transmit antenna ports in LTE, therefore, reference signals are transmitted from each antenna port. The reference signals on the second antenna are offset in the frequency domain by three sub-carriers, and to allow the UEs to accurately measure the radio link properties, nothing is transmitted on the other antenna at the same time-frequency location of reference signals.
The reference signals provide an amplitude and/or phase reference for allowing the UEs to correctly decode the remainder of the downlink transmission. In LTE (as distinct from LTE-A), reference signals can be classified into a cell-specific (or common) reference signal (CRS), an MBSFN reference signal used in MBMS, and a user equipment-specific reference signal (UE-specific RS).
The CRS is transmitted to all the UEs within a cell and used for channel estimation. The reference signal sequence, which spans the entire downlink cell bandwidth, depends on, or implicitly carries, the cell identity or “cell ID”. As a cell may be served by an eNodeB having more than one antenna port, respective CRS are provided for each antenna port and the locations of CRSs depend on the antennal port. The number and location of CRSs depends not only on the number of antenna ports but also on which type of CP is in use.
The MBSFN reference signal can be transmitted in sub-frames allocated for MBSFN transmission (see below).
A UE-specific reference signal is received by a specific UE or a specific UE group within a cell. UE-specific reference signals are chiefly used by a specific UE or a specific UE group for the purpose of data demodulation.
CRSs are transmitted in all downlink sub-frames in a cell supporting non-MBSFN transmission. If a sub-frame is used for transmission with MBSFN, only the first few (0, 1 or 2) OFDM symbols in a sub-frame can be used for transmission of cell-specific reference symbols.
CRSs can be accessed by all the UEs within the cell covered by the eNodeB regardless of the specific time/frequency resource allocated to the UEs. They are used by UEs to measure properties of the radio channel—so-called channel state information or CSI—with respect to such parameters as a Channel Quality Indicator, CQI.
LTE-A (LTE-Advanced) introduces further reference signals including a Channel State Information reference signal CSI-RS, and a UE-specific demodulation reference signal DM-RS, not to be confused with demodulation reference signals transmitted on the uplink by the UEs. These additional signals have particular application to beamforming and MIMO transmission techniques outlined below.
Resource Element Groups (REGs)
Resource Element groups (REGs) are blocks of consecutive REs within the same OFDM symbol. They are used to define the mapping of control channels (see below) to resource elements. Within each sub-frame, the REGs are located in the first few (usually four) symbols and have the same pattern in every sub-frame.
FIG. 3 shows the relationship between REGs and the REs in the first four symbols of one RB. In FIG. 3 and the subsequent Figures, the symbols are arranged along the horizontal axis and the subcarriers are arranged along the vertical axis. Here, only the first few symbols are shown for simplicity; however, it will be understood that each sub-frame actually contains ten slots each with Nsymb symbols as shown in FIG. 2.
Each REG is represented by an index pair (k′, l′) where k′ is the subcarrier index (numbered starting with 0 in this example) of the “first” RE within the REG, and l′ is the symbol index (again starting from 0) of the REG. In FIGS. 3, A, B, C, and D each denote REs allocated to a respective REG. As is clear from the Figure, each REG contains four REs.
More specifically, each REG contains four REs which are not already occupied by a CRS, as shown in FIGS. 4 and 5, where FIG. 4 illustrates the case of a normal CP and FIG. 5, the extended CP, both assuming the case of a one- or two-antenna port configuration. As shown here, all REs within the first four symbols are allocated to a REG; thus the boundaries of the REGs are dependent on where the CRS are located.
More particularly, as shown in FIG. 4 for example, CRSs are present within the first symbol (numbered with index 0). As each REG requires four unoccupied REs, the twelve REs in the first symbol of one RB form two REGs. In the second and third symbols (index 1 and 2) no CRS is present (in this antenna port configuration) so that each symbol can contain three REGs.
In the case of a four (or more) antenna port configuration, the arrangement is similar except that the CRS will now extend into the second symbol, so that only two REGs per RB are available in that symbol.
When the extended CP is used, only 12 symbols (instead of 14) are contained in each sub-frame. The effect of this is that CRSs need to be located in the fourth symbol as well as in the first symbol. In FIG. 5, the effect on RE allocation to REGs is shown. The difference is that now only two REGs per RB may be defined in the fourth symbol.
Channels
Several channels for data and control signalling are defined at various levels of abstraction within the network. FIG. 6 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them. For present purposes, the channels at the physical layer level are of most interest.
On the downlink, user data is carried on the Physical Downlink Shared Channel (PDSCH). There are various control channels on the downlink, which carry signalling for various purposes including so-called Radio Resource Control (RRC), a protocol used as part of the above-mentioned RRM. In particular the Physical Downlink Control Channel, PDCCH (see below).
Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH), and control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel quality indication (CQI) reports, precoding matrix information (PMI), a rank indication for MIMO (see below), and scheduling requests.
PDCCH, PCFICH and PHICH
Three control channels of particular interest in the present invention are PDDCH, PCFICH and PHICH.
PDCCH is used to carry scheduling information—called downlink control information, DCI—from base stations (called eNodeBs in LTE) to individual UEs. The PDCCH is located in the first few OFDM symbols of a slot as explained below. More particularly PDCCH contains:                the resource allocations for the downlink transport channel DL-SCH shown in FIG. 6        Transmit Power Control (TPC) commands for PUCCH and the uplink transport channel UL-SCH in FIG. 6; these commands enable the UE to adjust its transmit power to save battery usage        Hybrid-Automatic Repeat Request (HARQ) setup information        MIMO (see below) precoding information.        
A cyclic redundancy check (CRC) is used for error detection of the DCI. The entire PDCCH payload is used to calculate a set of CRC parity bits, which are then appended to the end of the PDCCH payload.
As multiple PDCCHs relevant to different UEs can be present in one sub-frame, the CRC is also used to specify which UE a PDCCH is relevant to. This is done by scrambling the CRC parity bits with a Radio Network Temporary Identifier (RNTI) of the UE. Other identifiers may also be applied.
The size of the DCI depends on a number of factors, and thus it is necessary that the UE is aware of the size of the DCI, either by RRC configuration or by another means to signal the number of symbols occupied by PDCCH.
PCFICH (Physical Control Format Indicator Channel) is used to indicate the number of symbols used in the current sub-frame for the PDCCH transmission. It is always located in the first OFDM symbol of each sub-frame (as initially the size of the downlink control information is unknown). This helps a UE to know where to look for the control information, and indirectly, for user data.
PHICH (Physical Hybrid-ARQ Indicator Channel) carries ACK/NACK for uplink data transmission. That is, the eNodeB sends a HARQ indicator to the UE to indicate a positive acknowledgement (ACK) or negative acknowledgement (NACK) depending on whether it has successfully received data from the UE on the uplink.
Not only the CRSs as outlined above, but also these control channels PDCCH, PCFICH and PHICH, are located within the first few symbols of each sub-frame. One advantage of this is to allow a UE to reduce power consumption by switching off its receiver if it is not scheduled for communication in the remainder of the sub-frame.
Referring to FIG. 7, this shows one example of how the above channels are mapped to REGs/REs within the first few (in this case, three) symbols. As before the horizontal axis represents symbol number (denoted in this case by an index l, numbered from 0) and the vertical axis represents the subcarriers (with index k, again numbered from 0 in this Figure).
Shaded REs are used for reference signals of each of four antenna ports in this example. REs marked C are used for PCFICH, and others marked H allocated to PHICH. REs labelled with numbers denote symbol quadruplets (corresponding to REGs) capable of being assigned for various purposes including PDCCH. The REGs marked by thick black lines are allocated to PDCCH in this example.
The following points are pertinent, as will become clear later:
(a) PDCCH may occupy the first 1, 2, 3 or 4 OFDM symbols in a sub-frame (4 is a special case for small system bandwidths).
(b) A given PDCCH may be transmitted in any one of a number of given locations (that is, a search space comprising a pre-determined subset of all the possible locations). The UE attempts blind decoding of the PDCCH in each location within the search space.
(c) PDCCH mapping to Resource Elements (REs) avoids conflict with CRS, PCFICH and PHICH, for which the locations generally depend on Cell ID and/or configuration.
(d) PCFICH is mapped to a set of REGs depending on Cell ID (PCFICH is only transmitted in the first OFDM symbol as already mentioned; PCFICH is not transmitted in MBSFN cells which do not support PDSCH).
(e) PHICH is mapped to a set of REGs not used for PCFICH, and therefore depends on Cell ID. PHICH starts in the first OFDM symbol; its duration may be 1, 2 or 3 OFDM symbols (thus, FIG. 7 is a simplified example); and PHICH cannot extend into PDSCH region (so the PDCCH duration is at least as great as the PHICH duration).(f) CRS are used as an amplitude and/or phase reference for reception of PDCCH.
To summarise, the same OFDM symbols are typically shared by PDCCH, CRS, PCFICH, and PHICH. The PDCCH is mapped to resources not used by the other channels.
MIMO
A technique called MIMO, where MIMO stands for multiple-input multiple-output, has been adopted in LTE due to its spectral efficiency gain, spatial diversity gain and antenna gain. One use of the MIMO technique is for so-called transmit (Tx) diversity, where blocks of data intended for the same UE are transmitted via multiple transmitting antenna ports, the signals from which may follow different propagation paths.
“Diversity coding” refers to the process for generating signals for transmission in a transmit diversity system. The antenna ports may be Tx antennas of different eNodeBs or of the same eNodeB. In LTE, owing to limitations on the physical size and capabilities of UEs, transmit diversity is more applicable on the downlink than to the uplink. Only one receiving antenna port (Rx antenna) is needed at the UE, although two or more Rx antennas may be used to improve performance.
Space Time Block Coding (STBC) and Space Frequency Block Coding (SFBC) are common methods of diversity coding. These methods are referred to as “open loop” diversity schemes since the transmitters do not have perfect knowledge of the transmission channel. Briefly, the distinction between these methods is that in STBC, different symbols are transmitted from different antennas at the same time; whereas in SFBC, the space coding is applied across neighbouring subcarriers within the same symbol such that different subcarriers are transmitted from different antennas at the same time.
CoMP and MBMS
Related to the above, it is possible to coordinate the MIMO transmissions among multiple base stations (i.e. coordinating transmissions in adjacent or nearby cells) to reduce inter-cell interference and improve the data rate to a given UE. This is called coordinated multi-point transmission/reception or CoMP, and is a technique being considered for inclusion in LTE-A. Downlink schemes used in CoMP include “Coordinated Scheduling and/or Coordinated Beamforming (CS/CB)” and “Joint Processing/Joint Transmission (JP/JT)”. An additional technique which may be employed is aggregation of multiple carriers (CA) to increase the available peak data rate and allow more complete utilisation of available spectrum allocations.
In CS/CB, data to a single UE is transmitted from one transmission point, but decisions regarding user scheduling (i.e. the scheduling of timings for transmissions to respective UEs) and/or beamforming decisions are made with coordination among the cooperating cells (or cell sectors). In other words, scheduling/beamforming decisions are made with coordination between the cells (or cell sectors) participating in the coordinated scheme so as to prevent, as far as possible, a single UE from receiving signals from more than one transmission point.
On the other hand, in JP/JT, data to a single UE is simultaneously transmitted from multiple transmission points to (coherently or non-coherently) improve the received signal quality and/or cancel interference for other UEs. In other words the UE actively communicates in multiple cells and with more than one transmission point at the same time. From the viewpoint of the UE, it makes no difference whether the cells belong to different eNodeBs or to the same eNodeB.
In CA, discrete frequency bands are used at the same time (in other words, aggregated) to serve the same user equipment, allowing services with high bandwidth demands (up to 100 MHz) to be provided. CA is a feature of LTE-A (LTE-Advanced) which allows LTE-A-capable terminals to access several frequency bands simultaneously whilst retaining compatibility with the existing LTE terminals and physical layer. CA may be considered as an complement to JP for achieving coordination among multiple cells, the difference being (loosely speaking) that CA requires coordination in the frequency domain and JP in the spatial domain.
FIG. 8 schematically illustrates the principles of CS/CB and JP downlink transmission schemes respectively, used in CoMP.
Joint Processing (JP) is represented in FIG. 8(a) in which cells A, B and C actively transmit to the UE, while cell D is not transmitting during the transmission interval used by cells A, B and C.
Of less relevance to the present invention, coordinated scheduling and/or coordinated beamforming (CS/CB) is represented in FIG. 8(b) where only cell B actively transmits data to the UE, while the user scheduling/beamforming decisions are made with coordination among cells A, B, C and D so that the co-channel inter-cell interference among the cooperating cells can be reduced or eliminated.
As another example of co-operative transmission among base stations, MBMS (Multimedia Broadcast Multicast Services) may be performed via multi-cell transmission. In case of multi-cell transmission, the cells and content are synchronized to enable for the terminal to combine the received signal from multiple eNodeBs. This concept is also known as a Single Frequency Network. The E-UTRAN can configure which cells are part of an Single Frequency Network for transmission of an MBMS service, so-called MBSFN operation. The MBMS traffic can share the same carrier with the unicast traffic or be sent on a separate carrier. For MBMS traffic, the above-mentioned extended CP is provided, allowing the UEs to combine the transmissions from the different eNodeBs, and in the case of sub-frames carrying MBSFN data, specific MBSFN reference signals are used as already mentioned.
Co-Ordination of Control Signalling Among Cells
In conventional multi-cellular networks, the spectral efficiency of downlink transmission is limited by the inter-cell interference. One approach to this problem is to coordinate the transmissions among multiple cells (which may imply multiple base stations) as already mentioned, in order to mitigate the inter-cell interference. As a result of the coordination (CoMP), the inter-cell interference can be reduced or eliminated among the coordinated cells, resulting in a significant improvement in the coverage of high data rates, the cell-edge throughput and/or system throughput. CoMP techniques may be applied to MIMO transmissions, although that is not a major consideration for the present invention.
Currently in LTE, a single control channel (PDCCH) is transmitted to the UE from one serving cell (the primary cell or Pcell). For a UE at the cell border, the transmissions from the Pcell suffer from increased interference from neighbouring cells operating at the same frequency, and typically a lower effective transmission rate is used to increase robustness to such interference. This may be applied for both data and control channel and can be achieved by lowering the code rate and/or repeating the message. Both approaches require more transmission resources. In the context of PDCCH transmission, the factor by which the resources are increased (in the frequency domain) is referred to as the aggregation level (which may take values of 1, 2, 4 or 8).
For at least some UEs (e.g. at the cell border) it would be beneficial to be able to jointly transmit the same PDCCH message from two cells. This would greatly improve the SINR for such a message and could allow aggregation level 1 to be used instead of 4, for example. In this case, for the same amount of resource, this could allow PDCCHs to be transmitted to 4 UEs instead of 2. However, there are some problems in doing this within the constraints of the current LTE specifications.
Neighbouring cells are typically given different cell IDs, which can be used as a basis for distinguishing transmissions from different cells. For example, data transmissions are scrambled by sequences which depend on the cell ID. The locations of the common reference symbols (CRS) in the frequency domain also depend on the cell ID. In practice neighbouring cells must have different cell IDs. One reason for this is so that the CRS occupy different locations, otherwise channel measurements for the different cells using CRS are not feasible if the OFDM symbols for CRS happen to be aligned in the time domain. The resources used by other channels such as PCFICH and PHICH also depend on the cell ID.
To achieve joint transmission of PDCCH from different cells would require that radio frames are time-aligned, so that the PDCCH regions overlap (in other words, that the frames, or at least sub-frames within a frame, have start and end timings which coincide within some tolerance). This would also mean that the CRS symbols overlap in the time domain, so different cell IDs become essential to allow different locations in the frequency domain. Therefore, the resources required for CRS, PCFICH and PHICH are in principle different between the different cells. Therefore, even with aligned radio frames, in general, different resources are used for two otherwise identical PDCCH messages in different cells.
Embodiments of the invention address the problem of providing common PDCCH resources between two co-operating cells with different cell IDs. An additional desirable feature of any solution is to ensure that any modification to PDCCH transmissions is backwards compatible with existing signals (i.e. the use of “old” PDCCH transmissions is not impacted by the simultaneous use of “new” PDCCH transmissions). However, as will become clear, the present invention is by no means restricted to use with PDCCH or even with control channels in general.